1. Field of the Invention
The present invention relates to methods and apparatus for improving the performance of multimode optical fibre communications systems, and in particular to methods and apparatus for use in connecting optical transceivers to multimode fibres.
2. History of the Prior Art
In the late 1970s, and early 1980s, much work was carried out to improve performance of multimode optical fibre communications systems. However, when multimode fibre was replaced by a singlemode fibre, as the medium of choice for use in high bit rate, long distance communications systems, much of this work ceased. Multimode fibre has continued to be used in optical communications for systems operating at lower bit rates, and over shorter distances, for example in building or campus LANs. Such multimode fibres are predominantly used in the LAN backbone but may also be used in horizontal links to users and appliances. There is thus a large installed base of multimode fibre, which represents a significant investment.
In recent years the demand for high data rate LANs has increased dramatically, for example to 1 GBit/s and beyond. The required data rates cannot be achieved utilising conventional techniques with LANs containing significant multimode fibre, even when the lengths of fibre are relatively short (500 meters).
A key aspect in determining the bandwidth of a multimode optical fibre communications link, which has been recognised for many years, is the number and distribution of modes within the multimode fibre which are excited, and therefore carry optical energy. See for example Chapter 7 of xe2x80x9cOptical Fibres for Transmissionxe2x80x9d by John E. Midwinter, published by John Wiley and Sons in 1979. If a pure low order single mode is launched into a multimode fibre, and there is no mode mixing, the bandwidth and other characteristics of the optical communication link will be that of a single-mode fibre, i.e. the link will have high bandwidth. If mode mixing occurs, for example due to fibre profile irregularities, or mechanical perturbations of the fibre, energy will be coupled from the single lowest order mode into higher order modes having higher group velocities, and additional pulse dispersion will inevitably result, leading to a lower overall bandwidth for the communications system. Alternatively, if light is launched into the same multimode fibre in a manner so as to uniformally excite all modes of the multimode fibre, and if no mode mixing occurs, a maximum pulse spread will be seen, and the bandwidth of the communications system will be at a minimum. If mode mixing is introduced to this situation, because individual photo ns will then spend some time in many different modes, and will have travelled many short distances at different group velocities, less pulse spreading will be experienced. In the ideal case rather than experiencing an increase of pulse spreading which is proportional to the length of the optical communications link, pulse spreading builds up only in proportion of the square root of the length of the optical communications link. Thus, in the early 1980s, although various alternative schemes were investigated (see eg U.S. Pat. No. 4,050,782 and U.S Pat. No 4,067,642), it was generally accepted that it was desirable to launch many modes into a multimode optical fibre, and to ensure that adequate mode mixing occurred in order to achieve a reasonable, and predictable, bandwidth for an optical communications link.
Despite this practical approach, it was however theoretically predicted that if the number and distribution of modes excited within a multimode fibre could be precisely controlled, the bandwidth of the communications link could be improved. For example, see Section 7.6, page 126 of Midwinter""s book where it is suggested that controlled mode coupling can be utilised to prevent coupling to the highest order modes of the fibre so as to increase the fibre bandwidth without incurring a loss penalty. Nevertheless, it is stated here that xe2x80x9cIt must be said, however that experimentally it looks extremely difficult to achieve such a precisely controlled fibre environment, and at the time of writing no reports of experimental testing are known.xe2x80x9d
In recent years lasers rather than LEDs (Light Emitting Diodes) have been utilised with multimode optical fibre communications systems. There are a number of reasons for this, of which the predominant one is that lasers can be directly modulated at higher speeds than LEDs. In contrast to LEDs, lasers can easily be utilised to excite only a few, low order modes of the multimode optical fibre. As discussed above, if only a few modes of a multimode fibre are excited, and little mode mixing occurs, the bandwidth of a multimode optical fibre communications system can in principle be increased somewhat. For example, data rates up to 1 GBit/s have been achieved over a maximum of 200 meters using a 780 xcexcm laser diode and 62.5 xcexcm multimode fibre.
There are significant differences between use of an LED and use of a laser in launch of a signal into multimode fibre. Characteristically, an LED launch will be an overfilled launch and hence will cause the modes of the multimode fibre to be fully populated. Bandwidth of multimode fibre is characterised according to its performance for such a launch. However, as indicated above, a laser does not have an overfilled launchxe2x80x94instead, there will be a a restricted launch in which only certain of the fibre modes will be partially populated or largely unpopulated. The nature of the restriction of the launch is dependent on a number of factorsxe2x80x94lower numerical aperture of the laser than the multimode fibre, smaller spot size than core diameter, nature of the laser source and coupling arrangement (constituents of the coupling mechanism such as lenses, fibre stubs etc.).
The present inventors have found that restricted launch into a multimode fibre can have a serious effect on the bandwidth achievable with the fibre, even where the bandwidth of the fibre is nominally in specification according to the overfilled launch bandwidth. The particular difficulty found is that the bandwidth exhibited by a fibre is strongly dependent on the details of the restricted launch. The actual bandwidth achieved can be significantly higher than the overfilled launch bandwidthxe2x80x94it can also be significantly lower. This creates a serious problem for system designers, as it is thus not possible to guarantee what minimum bandwidth will be encountered.
Accordingly, in a first aspect the invention provides apparatus for connecting an optical transceiver to multimode optical fibre in a multimode optical fibre communications system, the apparatus comprising: a transmission part adapted for receiving outgoing optical radiation admitted to the apparatus from an optical source of the optical transceiver at a radiation input into a single mode optical fibre of the transmission part and adapted for transmitting said outgoing radiation out into a first multimode optical fibre of the multimode optical fibre communications system after passage through the single mode optical fibre, and; a reception part adapted for receiving incoming optical radiation admitted to the apparatus from a second multimode optical fibre of the communications system into a multimode optical fibre of the reception part and adapted for transmitting said incoming radiation into a receiver of the optical transceiver.
This arrangement can be used very effectively to provide launching of light into multimode fibre with satisfactory bandwidth results. There are a number of advantageous possibilities with this arrangement. A particularly advantageous arrangement is for the length of single mode fibre to be sufficiently long that the light emitted from the fibre will be substantially single mode light. This approach solves the problem of reliable bandwidth assessment indicated above, because the launch of a single mode of radiation from a single mode fibre into a multimode fibre is well understood, and will guarantee that at least the overfilled launch bandwidth of the multimode fibre will be achieved on a consistent basis. Experimental results relating to launching of a single mode into a multimode fibre are discussed in xe2x80x9cA Mode-Filtering Scheme for Improvement of the Bandwidth-Distance Product in Multimode Fiber Systemsxe2x80x9d, Haas, Z. and Santoro, M. A., Journal of Lightwave Technology, Vol. 11, No. 7, July 1993. It is however necessary that the optical radiation is provided as substantially a single mode. Prior art transmitter structures are known incorporating a xe2x80x9cfibre stubxe2x80x9d of fibre, which may be single mode, after the laser (e.g. U.S. Pat. No. 5,315,680). These provide eye safety in the event of removal of a connector while the laser is functioning, as much light from other modes is stripped away by the cladding of the fibre stub, thus reducing the light emitted from the structure to safe levels. However, although these arrangements serve to remove significant amounts of the light from other modes, much too short a length of fibre is provided to limit the light emitted to substantially only single mode optical radiationxe2x80x94these arrangements hence do not solve the bandwidth problem solved by the present invention.
In certain embodiments, the apparatus comprises only a single mode fibre for launching light into the multimode fibre system. This may be achieved by a normal centre launch, but advantageously a mode conditioning means is provided in the transmission part such that chosen modes of the multimode fibre will be preferentially excited by the outgoing radiation so as to increase the operational bandwidth of the first multimode optical fibre of the communications system. This can be achieved where the apparatus has only a single mode fibre for launching light into the multimode fibre system by launching the outgoing radiation from the single mode fibre into the multimode fibre, such that the outgoing radiation illuminates an end face of the multimode fibre away from the axis of the multimode fibre.
In an alternative embodiment, the mode conditioning means is achieved by using a further multimode fibre together in the apparatus with the single mode fibre, such that light passes into the single mode fibre, into the further multimode fibre through the connecting means, and out into the first multimode fibre of the multimode fibre communications system. Accordingly, a reliable and effective launch into multimode fibre can be achieved.
It is advantageous if the multimode fibre length of the apparatus (for providing light to the receiver) has a core size greater than or equal to the core size of the second multimode fibre of the multimode fibre system. With this arrangement, light from the second multimode fibre does not have to make a transition to a smaller diameter fibre, which would give rise to both loss and modal noise.
This apparatus is advantageously embodied in a patchcord, although a dongle (with coiled fibres) is an alternative. Preferably, appropriate keying or other means is provided to ensure correct connection between optical source or receiver and the corresponding multimode fibre through the appropriate fibre length. As is discussed below, considerable technical and practical advantages are provided where the apparatus is in the form of a duplex patchcord comprising the single mode fibre length and the multimode fibre length. In practical s systems, a patchcord of some form will often be required in any event for connection between an optical transceiver and the installed fibres of an optical fibre communication system. Employing a patchcord in accordance with the invention for this purpose causes no change in practice for the user, but achieves a solution to the minimum bandwidth guarantee problem discussed above.
In a further aspect, the invention provides a communications device for use in an multimode fibre optical communications system, comprising an optical transceiver and an apparatus as indicated above. In a still further aspect, the invention provides a communications system comprising an optical transceiver and first and second multimode optical fibres for receiving outgoing optical radiation from and supplying incoming optical radiation to the optical transceiver respectively, the optical transceiver and the first and second multimode optical fibres being connected by a connecting apparatus, the connecting apparatus being an apparatus as described above.
In a yet further aspect, the invention provides a method of constructing a connection apparatus for connecting an optical transceiver to multimode optical fibre in a multimode optical fibre system, comprising: providing a length of single mode fibre terminating in a first ferrule, and providing a length of multimode fibre terminating in a second ferrule, wherein the wherein the second ferrule and the first ferrule are coaxial, wherein the single mode fibre and the multimode fibre are each mounted within the first ferrule and the second ferrule respectively such that the axis of each fibre is offset from the axis of the ferrule; rotating the first ferrule with respect to the second ferrule such that the single mode fibre is offset from the launch multimode fibre, measuring an output property of light from the multimode fibre, and fixing the first ferrule with respect to the second ferrule when a satisfactory value of the output property is achieved; providing a further length of multimode fibre, and adding connectors to the fibres, such that a first fibre path comprising the length of single mode fibre and the length of multimode fibre is formed for connection between a source of an optical transceiver and a first fibre of a multimode fibre network, and such that a second fibre path comprising the further length of multimode fibre is formed for connection between a receiver of the optical transceiver and a second fibre of the multimode fibre network.
In one further aspect, the invention provides a method for connecting an optical transceiver to multimode optical fibre in a multimode optical fibre communications system, comprising connecting the optical transceiver to first and second multimode optical fibres of the multimode optical fibre communications system with a connecting apparatus comprising a transmission part adapted for receiving outgoing optical radiation admitted to the apparatus from an optical source of the optical transceiver at a radiation input into a single mode optical fibre of the transmission part and adapted for transmitting said outgoing radiation out into a first multimode optical fibre of the multimode optical fibre communications system after passage through the single mode optical fibre and a reception part adapted for receiving incoming optical radiation admitted to the apparatus from a second multimode optical fibre of the communications system into a multimode optical fibre of the reception part and adapted for transmitting said incoming radiation into a receiver of the optical transceiver, wherein outgoing radiation is transmitted from an optical source of the optical transceiver through the transmission part of the connecting apparatus to a first multimode fibre of the multimode optical fibre communications system, and incoming radiation is transmitted from a second multimode optical fibre of the multimode optical fibre communications system through the reception part of the connecting apparatus to an optical receiver of the optical transceiver.